Method for crystal growth



Feb 16, 1932. 5 CQCKSEDGE 1,845,742

METHOD FOR CRYSTAL GROWTH Filed April 14, 1930 l atented Feb. 16, 1932 UNITED STATES PATENT- OFFICE HERBERT EDWIN CQUKSEDGE, F HARTFORD, ENGLAND, ASSIGNOB T0 IMPERIAL om- IC'AL INDUSTRIES LIMITED, 01 LONDON, ENGLAND, A BRITISH COMPANY METHOD FOR CRYSTAL GROWTH Application filed April 14, 1930, Serial No. 444,245, and in Great Britain April 18, 1989.

lit is known that to promote the growth of uiai. sized crystals from solution the Iiplb oi elutriation of the coarser cryscan be employed, and further that the ruction of the liner crystals or nuclei is 1uilvantageous and further that circulation of the magma in a continuous manner is advaningeous as tending to prevent the occurrence Ill supersaturated zones leading to the proluction of new nuclei spontaneously instead t desired growth upon surfaces already cut,

y invention relates to a suitable form of u crystalliaing apparatus in which crystalline owt-h is effected in the best manner possible, by a combination of these known principles in a novel manner in one and the same apparatus.

The apparatus in its preferred form comprises a vessel having an annular partition with means to circulate liquor down the central space and up the annular space and preferably having curved surfaces at the bottom. The invention further consists of the apparatus for effecting crystal growth more especially in combination with an elutriating tube as well as the improved process, all as hereinafter described, and more particularly defined in the appended claims.

The invention will be described by way of example with reference to the accompanying drawing in which a vessel 1 is provided with an internal cylinder 2 of such a suitable diameter that, having regard to the increased velocity due to stirring or gas lifting as described below, the relative areas of the central space and the annular space shall give equal speeds of travel of the liquor. A downward thrust is imparted by a propeller stirrer 3, or by pumping or similar means, and an upward thrust by change of direction aided by a suitably curved bottom of the vessel and if desired by the use of a gas stream, e. g, air for cooling or gases for a desired reaction, introduced at 4.

Such gas stream is not intended in any sense to lead to elutriation or segregation of particles, but is for the purpose of air coolm lug or reaction if desired, and in fact the separation of large from small crystals occurs outside the zone of air cooling, where air is employed. It is not essential however to use air or gas at all.

At the bottom of the vessel an elutriating tube 5 is attached with a suitably shaped connection portion, and up this tube is passed a stream of liquid (e. g. the solution to be evaporated or cooled which is supplied from a saturator 6 but clarified liquor may or may not be added by pumping) at such a rate determined for instance by the diameter of the tube, that only the coarser crystals which are continuously passing the top of the tube can descend owing to their weight to be collected in the vessel 7. Any lighter 05 particles that may chance to reach the upper portion of the tube are met by the ascending stream and carried back for renewed growth.

In this way there is a continuous and effective gradation between the crystals, and those which fall are in fact found to be remarkably uniform in size.

This described form of crystallizing vessel is (with a suitable design of the relative diameters of the cylinders concerned) very effective in crystallization, as there is no part in which liquid is not flowing quietly and continuously, and the relative movement of each crystal in virtue of its difierent density from the liquid is continuous, thus avoiding local supersaturation efiects with a consequent spontaneous precipitation of nuclei;

This avoidance of precipitation is so marked that in some cases one is able to add a controlled number of nuclei necessary for the production of a definite grade of crystal size, but in cases where owing to the nature of the substances in question (or of the reacting materials if any) there is an undesired precipitation of nuclei, I introduce the principle of destruction of such as follows The liquid is continually removed through a sieve 8 inserted in the vessel for instance at the side, carrying with it the unwanted nuclei which are destroyed by a slight reheating or addition of water or other means in the vessel 9 before returning the liquor through pump 10 and to saturator 6 to the vessel 1. It is only necessary to destroy such an amount as will maintain a mechanical formed, but it is obvious that the apparatus equilibrium of crystals to liquor to the desired degree, and in practice it is found that a ve s i ht reheating for instance will restore the ba ance. It is obvious that other means beside sieving may be employed, such as for instance an internal tube in which partial settling occurs or an outside settler or other means.

Again I am not restricted to the use of an elutriating stream for separation of the coarse crystals, though this is preferred; for instance the magma may be um ed over a shaking or rotating sieve pre erab y situated immediately over the top of the liquid in the crystallizing Vessel to remove such coarse crystals while returning the liquor and the rest of crystals for growth.

It is thus possible to combine three efiects all tending to enhance crystal size, viz.

(1) The continuous removal of large crystals when they have reached a desired size, thus encouraging deposition on smaller crystals and also leading to a satisfactory and automatically uniform product.

(2) The use of a carefully designed vessel in which the maximum of contact between crystal surfaces and liquor is effected with the minimum of abrasion which could cause as undesired seeding with new nuclei.

(3) The complete control of the desired size of product which contains saiy n nuclei per unit weight, by addition or estruction of nuclei so that the term 11. is rendered as constant as possible.

Example By application of the above princi les in combination, large perfect crystals 4 mm. long) of substances normally crystallizing from solution in motion in a relatively fine state, e. g. sodium carbonate monohydrate, have been formed with ease and substantially uniform in size.

I am not concerned in the above description with the method by Which the crystals are lends itself effectively to the growth of crystals by reaction or by cooling or by evaporation by the application of obvious and known means.

The apparatus further lends itself ideally to the continuous production of crystals by cooling a liquor which is at a substantially constant temperature, thus for instance giving a maximum yield of the salts per unit of cooling water.

I declare that what I claim is Process of crystallization which consists in circulating a solution through a crystal growth circuit in which growth takes place upon crystal nuclei and an auxiliary circuit in parallel therewith and removing the grown crystals through an elutriating zone, the growth circuithaving an annular space with an unobstructed interior to prevent abrasion Process as in claim 1 in which the auxiliaryi circuit contains means to destroy crystal nuc e1.

5. Process as in claim 1 in which the auxiliary circuit contains a heater to destroy crystal nuclei.

In witness whereof, I have hereunto signed my name this 4th da of April 1930.

HERBERT EnviuN oodKsEDeE. 

